[Curdle] Client-side SSH_MSG_EXT_INFO: Use it or lose it principle!

[denis bider <[e-mail]>]

Hey all:

it has come to my attention that at least one SSH server implementation (a)
advertises support for SSH_MSG_EXT_INFO as defined in RFC 8308, and (b)
disconnects on actual receipt of an EXT_INFO message from the client.

This happens when we define a general mechanism, but then the most widely
used implementations only use certain aspects of it. Lots of
implementations now support EXT_INFO sent by the server because it's needed
for RSA-SHA2 signatures, but only a handful implementations send EXT_INFO
by the client.

Bitvise SSH Client sends EXT_INFO if the server supports it. It's free to
use or test with - but it runs on Windows and lots of people don't want to
test with that.

It's possible that your implementation has no need for any of the
extensions that would appear in EXT_INFO from the client. However, if you
wish for this extensibility to be available in the future, then if your
client supports EXT_INFO, I would strongly suggest that it sends it.

If you want to send a client-side EXT_INFO that takes minimal effort, I
suggest:

1. Ensure that your SSH client can correctly handle SSH_MSG_GLOBAL_REQUEST
received at any point. Your client should not disconnect when it receives
this message, or act like it received a channel open failure if it receives
a global request when it's waiting for a channel open confirmation. These
are the two most common error modes relating to global requests.

2. Once you have ensured the above, if the server supports EXT_INFO, send a
client-side EXT_INFO implementing the extension "global-requests-ok" as
defined in this draft:

https://tools.ietf.org/html/draft-ssh-global-requests-ok-00

This tells the server that your client properly implements global requests
so that the server can use them for things like active keep-alives.

Implementing this would take minimal effort and would help ensure that
client-side EXT_INFO remains an available extension mechanism 10 years down
the road for SSH.

denis